During the recovery of hydrocarbons from subterranean formations, significant amounts of displaceable hydrocarbons are left behind because the reservoir strata are non-homogeneous. The natural drive fluids (e.g. brine or gaseous hydrocarbons) or secondary recovery flooding fluids (e.g. brine, steam, or carbon dioxide) flow through the more permeable strata of the reservoir resulting in progressively less hydrocarbon being recovered per unit volume of fluid produced. This increased ratio of drive or flooding fluid to hydrocarbon is usually referred to as early breakthrough of flooding fluid from injector wells or as excessive water encroachment in producer wells. It is desirable to increase the sweep efficiency, thereby producing more hydrocarbon per unit volume of fluid recovered. Chemicals have been used to achieve increased sweep efficiencies and to enhance hydrocarbon recovery by modifying the permeability of reservoir strata so that the natural or flooding fluids flow through hydrocarbon-rich strata in preference to strata that are more permeable and contain none or smaller amounts of recoverable hydrocarbons.
The chemicals that can be used to modify the permeabilities of subterranean reservoirs must be easily pumpable (i.e. nonviscous) so that they can be easily emplaced into the reservoir sufficiently far from the wellbore so as to be effective. It is desirable for the chemicals to reduce the permeability of the driving or flooding fluids while retaining most hydrocarbon permeability. Their emplacement should be selective in that they are emplaced into and reduce the permeability of reservoir strata more permeable to the driving fluids without seriously affecting the less permeable strata. Gels formed by polyacrylamide (U.S. Pat. No. 3,490,533) or polysaccharides with cations (U.S. Pat. Nos. 3,581,524; 3,908,760; 4,048,079) have been used as permeability modifiers for subterranean reservoirs. However, their application has been limited to subterranean reservoirs with ambient temperatures of less than about 70.degree. C. Major difficulties encountered in using gel forming chemicals in high temperature reservoirs (i.e.&gt;70.degree. C.) include:
(1) Increasing gelation rate with increasing temperature resulting in premature gel formation and therefore plugging of the reservoir strata nearest the wellbore with the needed permeability modification at substantial distances from the wellbore being unattainable
(2) Over-crosslinking and syneresis of the gel at elevated temperatures and in high total dissolved solid (TDS) reservoir brines which reduce the effectiveness of the gel as a permeability modifier.
(3) Decomposition by oxidative and/or hydrolytic mechanisms of polysaccharides and polyacrylamides at elevated temperatures with a net result of destruction of gel character. Some of the acrylamide groups of the polyacrylamide hydrolyze to carboxylic acid groups at elevated temperatures which causes the polyacrylamide to form calcium and magnesium salts resulting in precipitates which are undesirable.
Thus, there is a need for high temperature stable chemical compositions that are useful in reservoirs undergoing various fluid floods, especially those undergoing water floods with a variety of brines.